This invention relates to a binder composition and method for treating particulate material, to a method of preparing such binder composition, and to particulate material treated by the said method and binder composition.
UK patent number 1,184,129 (hereinafter referred to as D1) discloses a method and an aqueous solution for treating soil to improve its compressive strength and resistance to water permeation. The aqueous solution is mixed with the soil by being sprayed or poured onto the soil. The aqueous solution comprises:                a water-soluble urea-formaldehyde precondensate containing 0.2 to 6 percent unreacted formaldehyde based on the total weight of the said solution, the precondensate being present in the said solution in a concentration of 4 to 30% based on the total weight of said solution;        a setting agent chosen from organic and inorganic acids and acid salts thereof in an amount of 0.5 to 40% based on the total weight of the said solution; and        urea in an amount of 3 to 12 times the weight of the said unreacted formaldehyde.        
The preferred pH range of this solution is between 1 and 12 and as the pH of the solution decreases the setting time also decreases. In order to prevent the mixed solution from escaping through the soil to be treated, the setting time is adjusted to be as short as possible.
As is well known to those skilled in the art, an amino resin comprising an amine and an aldehyde is formed by two reactions, an addition reaction and a condensation reaction. When urea and formaldehyde are used, the addition reaction, or hydroxymethylation, comprises addition of formaldehyde to the binding sites of the urea, and the extent of the hydroxymethylation depends on the urea to formaldehyde ratio. Urea has four binding sites located on the two amine groups, with each having two binding sites. Thus, a maximum of four molecules of formaldehyde can bind to the binding sites of urea. The condensation reaction only takes place in acidic conditions and thus the time needed for the complete condensation reaction to take place decreases as the pH decreases. Where the reactions take place in a relatively shorter time, shorter chain polymers will form which will give the amino resin a more crystalline texture.
A disadvantage of the known solution is that, since the setting time is adjusted to be as short as possible, the solution has to be mixed and applied to the soil to be treated in a relatively short amount of time, thus making it difficult to treat large areas such as roads. Relatively short polymeric chains are also formed in the rapid polymerisation reaction.
Another disadvantage of the aqueous solution is that it is not suitable for mixing with bulk soil, such as in excess of 20 tonnes and applying the mixture to a road surface in a layer over a period of a working day and thereafter compacting the layer, as the aqueous solution starts setting before the entire mixture has been applied to the road surface and compacted.
The result thereof is that the known aqueous solution is only suitable for spraying onto the road surface, with the accompanying disadvantage that the penetration of the solution into the road surface is limited. The known solution therefore seals the top surface of the road, but does not stabilise the substrate.
UK patent no 1,194,227 (hereinafter referred to as D2) discloses A process for stabilising a water permeable soil and rendering it substantially water impermeable, including the steps of injecting into the soil an aqueous solution of urea, formaldehyde and polyvinyl alcohol, all in the form of water soluble water condensation products; and insolubilising the mixture in situ by the action of an acidic curing agent for urea-formaldehyde resins.
A disadvantage of the process disclosed in D2 is that it is a relatively very fast reaction so that the mixture sets in a short time from 1.7 minutes to 8 minutes. This renders the process of D2 totally unsuitable for any application, such as road building, where relatively longer setting times are required.